1. Field of the Invention
The present invention relates to an electrophotographic color toner having low resistance, capable of forming images excellent in image quality and chroma and causing no failure of the image forming device and the like to be used, and relates to electrophotographic color developer, a toner cartridge, an image forming device and an image forming method using the same.
2. Description of the Related Art
Electrophotography is the technology widely used in the image forming devices such as a copying machine, an electrophotographic facsimile, and an electrophotographic printer. As the electrophotography, the system using a photoconductive insulator, for example, as described in U.S. Pat. No. 2,297,691, is generally used. In this system, the photoreceptor charged with corona discharge or a charge-supply roller is first irradiated with a light such as laser, LED and the like to form an electrostatic latent image thereon. Next, resin particles (toner) colored with pigments and dyes are electrostatically deposited on the electrostatic latent image to develop to obtain a visualized toner image. Subsequently, the toner image is transferred to a recording medium such as paper or film. The toner image is a powder image which is only placed on the recording medium at this stage, so that it needs to be fixed on the recording medium. In the last step, the toner image is fused with heat, pressure, light or the like and then solidified on the recording medium to finally obtain a toner image fixed on the recording medium.
In the fixing of the toner image, a powder toner containing a thermoplastic resin (called also as a “binder resin” below) as a main component is fused with heat to be fixed on the recording medium. Generally known fixing systems include a heat-roll system in which a recording medium on which a toner image is formed is directly heated and pressed with a roller and a flash fixing system in which the toner is fixed on the recording medium with the irradiation of flash light such as a xenon flash lamp.
Fine particles of resins in which a colorant or the like is dispersed in a binder resin have conventionally been used as the toner for use in the electrophotography. However, the properties of such toner itself such as storage stability, a flowability property, a developing property, transferability and chargeability are not sufficient. Therefore, when the toner is used, additives are often added to the toner for the purpose of improving these properties.
The additives proposed include fine particles of hydrophobic silica (Japanese Patent Laid-Open No. 56-128956), silica fine particles which are added with aluminum oxide or titanium oxide fine particles (Japanese Patent Laid-Open No. 60-238847), titanium oxide by a vapor-phase process which is subjected to hydrophobic treatment (Japanese Patent Laid-Open No. 58-1157 and No. 59-52255), anatase-type titanium oxide (Japanese Patent Laid-Open No. 60-112052), aluminum oxide-coated titanium oxide (Japanese Patent Laid-Open No. 57-79961), and titanium oxide synthesized by a wet-process (Japanese Patent Laid-Open No. 07-175258 and No. 07-225489). In addition, the technology for treating the surface of inorganic compounds to be used for external additives such as titanium oxide fine particles whose surface is treated with a coupling agent (Japanese Patent Laid-Open No. 04-40467 and No. 04-348354) is also proposed.
The development in the electrophotography uses the developer prepared by mixing colored resin particles that has chargeability and is called toner with magnetic carriers that frictionally charge the toner. A typical black toner generally contains carbon as a colorant. The carbon is conductive and reduces electrical resistance of the toner, so that the developing property is improved; the edge effect is prevented; and the excessive charging of the toner during continuous printing can be prevented.
However, as a color toner cannot incorporate carbon that is black, it has higher electrical resistance compared with a black toner, which results in a problem of easily causing failure in development.
Conventional color toners have conductive fine particles as an additive adhered to the toner surface to reduce the electrical resistance to improve the developing property. However, the conductive fine particles are adhered to the toner only electrostatically, so that there has been a problem that the conductive fine particles adhered to the toner are easily detached from the toner when the toner is mixed with the carrier. Once the conductive fine particles are detached, the electrical resistance of the toner increases to change the properties of the developer, causing a problem of preventing stable development.
In addition, there have been problems that the conductive fine particles detached during agitation cause image degradation by the leak of toner charge and also adhere to a charging device or a transfer device to cause the failure or the like of devices and components by the leak of high voltage.
Various technologies have been developed in which the conductive fine particles proposed as the external additives are internally added in the toner to reduce the resistance. However, for obtaining appropriate electrical resistance, the quantity to be added needs to be 20 weight % or more. If the conductive fine particles are added in 20 weight % or more, the toner could become opaque to reduce the chroma of the toner.
In addition, when the powder having a lower electrical resistance of 1 Ωcm or less compared with the conductive fine particles proposed as external additives is internally added in the toner, it has the effect to reduce the electrical resistance by an added quantity of only 20 weight % or less. However, the resistance is reduced excessively to cause the charge held by the toner to leak when the powder is exposed to the toner surface, resulting in a problem of preventing a stable electrostatic property.
Further, fine particles having a size from several nm to several μm are typically used for the conductive fine particles having low electric resistance. Although most of these fine particles exist as primary particles, they exist as the aggregate having a size of 10 μm or more. Such a large aggregate extremely reduces the electrical resistance of the toner, resulting in the problems of causing image degradation by the leak of toner charge and the failure or the like of a charging device or a transfer device by the leak of high voltage.
In particular, in the electrophotographic devices that conduct high speed printing having a printing speed above 800 mm/s, the agitation stress that is given to the toner is extremely large, dramatically increasing the quantity of the toner scattered with the conveying of developer. Consequently, such devices require much higher performance to work out the above described various problems than the relatively low-speed electrophotographic devices do.